In a Long Term Evolution (LTE for short) system, an orthogonal frequency division multiplexing (OFDM for short) technology is used. In a system in which the OFDM technology is used, a direct current subcarrier (DC subcarrier for short) is located at a 0 Hz subcarrier location of a baseband signal.
According to different frequency bandwidths, carriers in the LTE system are corresponding to different transmission bandwidths. The transmission bandwidth is represented by using a quantity of physical resource blocks (PRB for short), one PRB includes 12 consecutive subcarriers in a frequency domain, and each subcarrier spacing is 15 KHz. One PRB includes six or seven consecutive OFDM symbols in a time domain. For a normal cyclic prefix (Normal CP for short), one PRB includes seven OFDM symbols; for an extended cyclic prefix (Extended CP for short), one PRB includes six OFDM symbols. A frequency-domain width of one PRB is 180 KHz, and a time length is 0.5 milliseconds (ms). A DC subcarrier is located at a center of an entire carrier frequency band of a carrier in the LTE system, and the DC subcarrier does not belong to any PRB of a transmission bandwidth.
For a device in the LTE system, there are two design manners: zero intermediate frequency and non-zero intermediate frequency. The zero intermediate frequency means that intermediate frequency modulation is not performed on an analog baseband signal, and a radio frequency signal is generated after one-time up conversion, or means that intermediate frequency conversion is not performed after a radio frequency signal is received, and an analog baseband signal is directly obtained by means of down conversion. The non-zero intermediate frequency means that at least one level of an intermediate frequency conversion process exists in a process in which up or down is performed between an analog baseband signal and a radio frequency signal. During up conversion or down conversion, a local-frequency signal is generated by a radio frequency oscillator, and may be leaked because the radio frequency oscillator has an extremely high frequency, that is, a high-frequency output signal of the radio frequency oscillator may be leaked or radiated to an input end of a device. After down-conversion demodulation is performed, the leaked signal generates an additional direct current component, and the additional direct current component interferes with frequency-domain information that is of a normal radio frequency signal and that is mapped on a DC subcarrier.
Generally, a receive end of a device that uses the zero intermediate frequency design is greatly interfered with by the direct current component of the local-frequency leakage. However, because an intermediate frequency conversion process needs to be performed for a device that uses the non-zero intermediate frequency design, an intermediate frequency circuit needs to be added; therefore, a receive end of the device that uses the non-zero intermediate frequency design is less interfered with by the direct current component of the local-frequency leakage. The direct current component of the local-frequency leakage causes little interference to both transmit ends of the device that uses the zero intermediate frequency design and the device that uses the non-zero intermediate frequency design. Generally, user equipment (UE for short) has a limited size and is sensitive to costs, and therefore, the UE generally uses the zero intermediate frequency design. A base station side device is less sensitive to a size and costs, and therefore, the base station side device generally uses the non-zero intermediate frequency design. That is, generally, a downlink DC subcarrier received by the UE is greatly interfered with.
In the LTE system, to resolve the problem that a downlink DC subcarrier may be interfered with, a DC subcarrier on a downlink carrier is vacated and not used, that is, the DC subcarrier is vacated and does not carry wanted data or a wanted signal. The DC subcarrier does not belong to any PRB of a carrier frequency band. On an uplink carrier, considering impact on a cubic metric (CM for short) or a peak-to-average power ratio (PAPR for short), a solution in which a vacated subcarrier is reserved is not used; instead, an entire frequency band is shifted by +/−7.5 KHz. In this way, no center of any carrier is right located on the DC subcarrier, and interference to data transmission on the uplink subcarrier is reduced.
In the LTE system, when accessing a base station, the UE first needs to detect information such as a primary synchronization signal (PSS for short), a secondary synchronization signal (SSS for short), and a physical broadcast channel (PBCH for short). A cycle of sending the PSS and the SSS is five subframes, and the PSS and the SSS occupy 72 subcarriers that are symmetric with respect to the DC subcarrier at the center of the frequency band. In the time domain, the PBCH occupies the first four symbols in the second time slot in a subframe 0, and in the frequency domain, the PBCH is also located on the 72 central subcarriers of the carrier frequency band. The PBCH carries a downlink carrier bandwidth. Before detecting the PBCH, the UE can identify only a downlink bandwidth having a frequency-domain width of 72 central subcarriers of a currently detected carrier, that is, all the PSS, the SSS, and the PBCH need to be placed within the downlink bandwidth having the frequency-domain width of the 72 central subcarriers of the carrier. The frequency-domain width corresponding to the 72 subcarriers that include the PSS, the SSS, and the PBCH may be referred to as an access bandwidth.
Because the access bandwidth is corresponding to the 72 subcarriers that use the DC subcarrier as a center, all UEs need to perform access at the center of the carrier. With evolution of the LTE system, in a scenario in which micro base stations are densely deployed, such an access manner causes more severe interference between common control channels such as PSSs, SSSs, and PBCHs of all cells, and the UE has higher difficulty in reading the foregoing common control channel or even cannot obtain the foregoing common control channel. Consequently, access of the UE is affected.